1. Field of the Invention
This invention relates to a process and catalyst for converting methane in the presence of oxygen into hydrogen and higher hydrocarbons which include ethane and ethylene.
2. Description of the Previously Published Art
Methane is a plentiful hydrocarbon feedstock which is obtained principally from natural gas. The methane content of natural gas can vary from 60% to 99%, the other components being ethane, propane, butane, carbon dioxide, and nitrogen. World reserves are estimated to be about 2.5.times.10.sup.12 ft.sup.-1. The production of chemicals from methane, however, is hampered by the lack of catalytic processes capable of activating methane towards chemical transformations. Today methane can be either combusted for its heating value, or steam reformed over iron or nickel catalysts to produce CO and H.sub.2. The CO and H.sub.2 are further reacted with N.sub.2 to produce methanol and ammonia. As yet no attractive processes exist to convert methane directly into higher valued hydrocarbons, such as ethylene or propylene, which can then be used to produce liquid fuels, plastics, fibers, solvents, and a myriad of other organic compounds used by the chemical process industry. As a consequence, methane is an underutilized natural resource.
U.S. Pat. Nos. 4,172,810, 4,205,194, and 4,239,658 disclose a novel catalyst and process for converting methane into a hydrocarbon product rich in ethylene and benzene. The essential components of the catalyst are
(1) a group VIII noble metal having an atomic number of 45 or greater, nickel, or group Ib noble metal having an atomic number of 47 or greater,
(2) a group VIa metal oxide which is capable of being reduced to a lower oxide, and
(3) a group IIa metal selected from the group consisting of magnesium and strontium composited with a passivated, spinel-coated refractory support or calcium composited with a passivated, non-zinc containing spinel-coated refractory support. The process consists of contacting the catalyst with methane at elevated temperatures for a short period of time, and recovering the hydrocarbons which are produced. During the exposure to methane some of the metal oxides contained in the catalyst are reduced, and the surface of the catalyst becomes covered with coke, rendering it inactive. Before the methane can be readmitted to the reactor, the catalyst must be regenerated by contact with an oxygen or water containing gas at elevated temperature.
U.S. Pat. No. 4,450,310 discloses a process for the conversion of methane into olefins and hydrogen by passing methane in the absence of oxygen and in the absence of water over a catalyst at temperatures above 500.degree. C. The catalyst is composed of mixed oxides from group IA of the periodic table, including Li, Na, K, Rb, and Cs, and group IIA of the periodic table, including Be, Mg, Ca, Sr, and Ba, and optionally a promoter metal selected from Cu, Re, W, Zr, and Rh. The improvement of this process over the one described previously is the reduced amount of coke deposited on the catalyst during reaction, which presumably allows the reaction to proceed for a longer period of time before the catalyst must be regenerated.
G. E. Keller and M. M. Bhasin in J. Catal. 73, 9 (1982) disclose a process to produce ethylene and ethane from methane whereby pure methane is fed over a catalyst at atmospheric pressure and temperatures of 500.degree. to 1000.degree. C. As the methane is passed through the reactor it reacts with the metal oxide catalyst producing ethane and ethylene, while simultaneously reducing the metal oxide. After a short exposure to methane, the catalyst must be regenerated, and the feed is switched to pure oxygen which reoxidizes the metal oxide. All catalysts were prepared by supporting a metal oxide on Al.sub.2 O.sub.3. Active catalysts are supported oxides of Sn, Pb, Sb, Bi, Tl, Cd, and Mn. The authors suggest that the ability of these oxides to cycle between two oxidation states is essential for having good activity and selectivity for the conversion of methane into C.sub.2 hydrocarbon products.
Jones, Leonard, and Sofranko issued a series of patents which disclose a process similar to that of Keller and Bhasin for producing ethane and ethylene from methane. The reaction is carried out at atmospheric pressure, temperatures of 500.degree. to 1000.degree. C, and the methane and oxygen are fed separately in a cyclic fashion. The improvements of the Jones process over the Keller process appear to be: (1) the use of a fluidized-bed catalytic reactor, instead of a fixed-bed catalytic reactor, and (.sub.2) supporting the active metal oxides on SiO.sub.2, instead of Al.sub.2 O.sub.3. Jones, Leonard, and Sofranko also suggest that "reducible" metal oxides must be used as catalysts for this process, and these are oxides of Sb, Mn, Ge, Pb, Sn, In, and Bi. These oxides are claimed in U.S. Pat. Nos. 4,443,644; 4,443,649; 4,443,645; 4,443,647; 4,444,984; 4,443,648; and 4,443,646, respectively. The reducible oxides can also be promoted with alkali metals (U.S. Pat. No. 4,499,322) or alkaline earth metals (U.S. Pat. No. 4,495,374) and stability is enhanced by the presence of phosphorus (PCT Published Application WO 85/00804). Other related work includes Ru promoted by alkali and alkaline earth metals (U.S. Pat. No. 4,489,215), and the use of reducible rare earth oxides, CeO.sub.2 (U.S. Pat. No. 4,499,324) and Pr.sub.6 O.sub.11 (U.S. Pat. No. 4,499,323).
Hinsen and Baerns (German Patent No. 3,237,079, Chem.-Ztg. 107, 223 (1983) and Proc. 8th Intl. Cong. Catal. 3, 581 (1984)) disclose a new process for the synthesis of ethylene and ethane from methane. The improvement of this process over previous processes is that methane and oxygen are fed simultaneously to the catalytic reactor, thereby obviating the need to cycle between reaction and catalyst regeneration. The preferred method of adding the oxygen is either laterally along the length of the reactor, or to a large recirculating stream of the hydrocarbon gas. These methods of oxygen addition insure that the oxygen partial pressure is kept low, so as to maximize selectivity. Good selectivities are observed at reaction temperatures of 650.degree. to 750.degree. C. and at low O.sub.2 partial pressures (P.sub.02 =0.05 to 0.10 atm) relative to methane (P.sub.CH4 =0.25 to 0.50 atm). For the continuous feeding case, Baerns found that reducible oxides of Pb, Sb, Sn, Bi, Cd, Tl, and In are active and selective catalysts. Reducing the acidity of the support is also essential for maintaining good selectivity, and this can be achieved by using SiO.sub.2 support instead of TiO.sub.2, SiO.sub.2 /Al.sub.2 O.sub.3, or Al.sub.2 O.sub.3, by increasing the PbO weight loading above 10.0%, and by promoting the catalyst with alkali.
U.S. Pat. No. 2,020,671 discloses the production of oxygenated organic compounds by reaction of methane with steam at temperatures of 200.degree.-700.degree. C. in the presence of catalysts selected from metal salts of the alkaline earth metals, aluminum, magnesium, and zinc.
U.S. Pat. No. 2,859,258 discloses the production of ethylene from methane in the presence of oxygen containing metal compound wherein the metal is selected from the second, third, and fourth groups of the periodic table, such as aluminum oxide, magnesium aluminum silicate, and magnesium aluminum molybdate.
3. Objects of the Invention
It is an object of this invention to obtain a catalytic process to convert methane in the presence of oxygen to hydrogen, ethylene, ethane, and higher hydrocarbons with high selectivities and high methane conversion per pass.
It is a further object of this invention to develop an active and selective catalyst for the synthesis of hydrogen, ethylene, ethane, and higher hydrocarbons from methane in the presence of oxygen.
It is a further object of this invention to operate the process with simultaneous addition of methane and oxygen to the catalytic reactor, so as to avoid intermittent regeneration of the catalyst, thereby obtaining a continuous synthesis of hydrogen, ethylene, ethane, and higher hydrocarbons from methane.
These and further objects will become apparent as the description of the invention proceeds.